IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 24, DECEMBER 15, 2006 2587

Stable Dual-Wavelength DFB Fiber Laser WithSeparate Resonant Cavities and Its Application

in Tunable Microwave GenerationJie Sun, Yitang Dai, Xiangfei Chen, Yejin Zhang, and Shizhong Xie, Senior Member, IEEE

Abstract—A single-longitudinal-mode dual-wavelength dis-tributed feedback fiber laser with a wavelength spacing of0.312 nm is proposed and demonstrated. Based on two spatiallyseparated resonant cavities in a single fiber Bragg grating madeby a simple method, stable dual-wavelength lasing is established.Then, a 38.67-GHz microwave signal generated by beating thetwo lasing wavelengths is obtained with a 3-dB bandwidth of

6 kHz and a frequency drift 5 MHz without any feedbackmechanism. As a potential application of this device, a tunablemicrowave source ranging from 18.67 to 58.67 GHz (with a smalldiscontinuity) is proposed and partially demonstrated.

Index Terms—Distributed feedback, dual-wavelength fiber laser,fiber Bragg grating, microwave generation.

I. INTRODUCTION

ALL-FIBER multiwavelength lasers have attracted muchattention because of their potential applications in fiber-

optic sensors, optical spectroscopy, and microwave photonicssystems. Among various multiwavelength fiber lasers [1]–[3],distributed feedback (DFB) fiber lasers exhibit many attractivefeatures such as considerable stability, compact size, single lon-gitudinal mode, and narrow linewidth. But the major obstacle toachieve stable multiwavelength oscillation is mode competitioncaused by the homogeneous broadening of the gain medium,e.g., Er : Yb codoped fiber (EYDF). Several approaches havebeen proposed to overcome this constraint, e.g., the spatial holeburning [4] and distributed Fabry-Pérot cavities in a stronglychirped fiber Bragg grating (FBG) [5].

In this paper, a novel stable dual-wavelength DFB fiber laserstructure is proposed and demonstrated. In this structure, lasercavities corresponding to the two lasing wavelengths are spa-tially separated to prevent mode competition from happening.The structure is based on two superimposed -phase-shifteduniform sub-FBGs with different center wavelengths. A simplefabrication method is also proposed to realize this structure.Then a stable dual-wavelength DFB fiber laser oscillating at1552.32 and 1552.632 nm is achieved. The total output poweris 830 W with a 980 nm/120 mW pump laser. A 38.67-GHzbeating signal is generated by the two wavelengths. The 3-dB

Manuscript received June 19, 2006; revised October 18, 2006. This work wassupported by the National Science Foundation Council of China (90604026).

J. Sun, Y. Zhang, and S. Xie are with the Broadband Optical Network Re-search Laboratories, Department of Electronic Engineering, Tsinghua Univer-sity, Beijing 100084, China (e-mail: jie-sun@mails.tsinghua.edu.cn).

Y. Dai and X. Chen are with the Broadband Optical Network Research Lab-oratories, Department of Electronic Engineering, Tsinghua University, Beijing100084, China, and also with the Microwave Photonics Technology Laboratory,Nanjing University, Nanjing, 210093, China.

Digital Object Identifier 10.1109/LPT.2006.887336

Fig. 1. (a) Index modulation of the sub-FBGs. (b) Total index modulation (grayline), sinusoidal apodization (solid line), and square-wave apodization (dashed-dottted line) of the FBG. (0, �, and��=2 indicate phases for each FBG region).

bandwidth of the electrical signal is 6 kHz and the frequencydrift is 5 MHz at room temperature without any feedback. As apotential application of the proposed laser, a tunable microwavesource ranging from 18.67 to 58.67 GHz (with a small discon-tinuity at 38.67 GHz) is proposed and partially demonstrated.

II. LASER STRUCTURE AND EXPERIMENT

The proposed laser structure is constructed by two superim-posed -phase-shifted sub-FBGs with different center wave-lengths and , as shown in Fig. 1(a). The two -phase-shifts,locating at and in the two sub-FBGs respec-tively, are spatially separated to form two partially separate res-onant cavities (this issue will be discussed in detail later on).Here, is the coordinate along the grating. Conventionally, thesuperimposed FBG shown in Fig. 1(a) can be achieved by twowriting steps at the expense of complicated fabrication and dif-ficulty in the precise control of the wavelength spacing. In thispaper, a simple fabrication method is used to avoid these short-comings.

In such a grating with two superimposed sub-FBGs, the totalindex modulation [gray line in Fig. 1(b)] can be ex-pressed by adding up those of the two sub-FBGs:

(1)

where is the magnitude of the index modulation and is thegrating period determined by the phase mask; denotes the

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2588 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 24, DECEMBER 15, 2006

Fig. 2. (a) Experimental transmission spectrum of the FBG (inset: calculatedspectrum). (b) Power distributions of the two lasing wavelengths.

complex conjugate. is determined by the wavelength spacingof the two sub-FBGs

(2)

where is the effective refractive index of the grating. It canbe seen from (1) and (2) that the superimposed FBG can beachieved by a single writing step using a sinusoidal apodizationwith period [black solid line in Fig. 1(b)]. Unfortunately, thisis difficult to realize especially when large wavelength spacingis required, since the apodization period is small. However,as st-order sinusoidal components of the square-wave aredominant, the index modulation in (1) can be achieved by usingsquare-wave apodization with period [dashed-dotted line inFig. 1(b)] instead of sinusoidal apodization. Then, the indexmodulation is given by

(3)

In (3), positive or negative real value in the apodization profilecorresponds to grating phase of 0 or , respectively, while pos-itive or negative imaginary value denotes grating phase ofor . These phases can be fabricated by shifting the phasemask mounted on a piezo-electric transducer (PZT).

In our experiment, the FBG is written on a piece of EYDFwith the proposed method utilizing a uniform phase mask. TheEYDF has an absorption coefficient of 10 dB/m at 980 nm, again coefficient of 15 dB/m at 1550 nm, and a numerical aper-ture of 0.22. The length of the FBG is 106 mm, with a gratingperiod nm. The wavelength spacing is 0.312 nm(38.67 GHz), and is 5.37 mm according to (2). The two

-phase-shifts locate at mm and mm,respectively. And the index modulation magnitude of theFBG is . Fig. 2(a) shows the experimentaltransmission spectrum of the FBG, while the calculated resultis shown in the inset. There are two transmission peaks inthe calculated grating stop bands, but they are too narrow toobserve by the optical spectrum analyzer (OSA). It is importantto point out that the local lasing power in such a DFB fiber

Fig. 3. Laser spectrum of the dual-wavelength fiber laser (inset: spectra mea-sured every 30 min for 3.5 h).

Fig. 4. Beating signal observed by an ESA (frequency span = 1 MHz, fre-quency resolution = 2 kHz).

laser concentrates around the position where the -phase-shiftoccurs. As the two -phase-shifts are spatially separated intwo sub-FBGs [Fig. 1(a)], the overlap of power distributionsbetween these two lasing wavelengths is small [Fig. 2(b)].Hence, two partially separated resonant cavities are formedand mode competition is thus significantly suppressed. In otherwords, gains of the two lasing wavelengths are provided bydifferent parts of the EYDF, greatly benefiting the stabilityof dual-wavelength operation. The FBG is then pumped bya 980 nm/120 mW laser diode to form a dual-wavelengthDFB fiber laser with a total output power of 830 W. And thelasing power can be enhanced simply by increasing the cavitylength. The lasing spectrum of the fiber laser is shown in Fig. 3(inset: laser spectra measured every 0.5 hr for 3.5 hrs), and thetwo lasing wavelengths are consistent with the transmissionpeaks in Fig. 2(a). Both wavelengths stably oscillate at singlelongitudinal mode attributed to the DFB structure and the shortcavity length. And they are both single polarization with thesame polarization state.

Since the two cavities are formed by a single FBG, the twolasing wavelengths have a fixed phase relationship and a goodcoherence despite the partially separated laser cavities. Hence,a stable microwave signal generated by beating the two wave-lengths is then obtained at 38.67 GHz by sending the laser outputinto a photodetector (PD). The signal is observed by an electricalspectrum analyzer (ESA), as shown in Fig. 4. The 3-dB band-width of the microwave signal is 6 kHz and the frequency driftis 5 MHz (observed for an hour) at room temperature withoutany feedback owing to the considerable stability and coherenceof the two lasing wavelengths.

III. APPLICATION

A possible application of such a dual-wavelength fiberlaser is the tunable microwave generation. Combining thedual-wavelength laser and previous microwave generation

SUN et al.: STABLE DUAL-WAVELENGTH DFB FIBER LASER 2589

Fig. 5. Setup of the tunable microwave source. C: circulator, FBG: fiber Bragggrating, IM: optical intensity modulator, BPF: optical bandpass filter, PD: pho-todetector. Components in dashed line: not used.

technique in [6], a tunable microwave source operating at an ar-bitrary working frequency band can be achieved. The schematicdiagram of the device is shown in Fig. 5. The dual-wavelengthfiber laser oscillates at frequencies and . Thetwo wavelengths are then modulated by a microwave signalwith frequency of in an optical intensity modulator (IM).The direct current (dc) bias of the IM is carefully adjustedat (half-wave voltage of the modulator) to suppress evenorder sidebands [6]. A precise and robust sliding rheostat isalso used in the modulator to mitigate dc bias drift. Then

st-order sidebands of the two fundamental frequencies aregenerated at and . The dispersion effect inthe optical distribution of these signals is negligibly small foroptical component separations in the tens of gigahertz regionand the fiber length of several tens of kilometers [6]. These fourharmonics are heterodyned in a PD and then beating signals at

are simultaneously obtained at the output.Moreover, a circulator and an optical filter with a reflectionband from to (e.g., the one in [7]) can be used to separatethem. Then frequency is achieved at PD1,while is generated at PD2. Considering thesteepness of the filter roll-off and the jitter of carrier frequen-cies, cannot be too small (e.g., GHz). Utilizingthe dual-wavelength fiber laser with GHz, aconventional tunable microwave source ( GHz),a microwave source with a tuning range of 18.67–35.67 GHzand 41.67–58.67 GHz can be achieved. In our experiment, sig-nals from 18.67 to 35.67 GHz are observed (Fig. 6), and signalshigher than 40 GHz, cannot be shown due to the bandwidthlimitation of the ESA. The 3-dB bandwidth of the microwavesignals in the whole tuning range is 10 kHz and the frequencydrift is 5 MHz. And the device can operate at an arbitraryworking frequency band simply by changing the wavelengthspacing . Moreover, if several such dual-wavelengthfiber lasers with different wavelength spacing are utilizedtogether in this device, it can cover a very wide tuning range.

IV. CONCLUSION

A novel dual-wavelength DFB fiber laser based on twopartially separate resonant cavities is proposed and experimen-tally demonstrated. The mode competition, which is the majorobstacle to achieve stable multiwavelength oscillation in fiberlasers, is successfully suppressed. A simple method to fabricatethis laser structure with a single FBG writing step without anycomplex apodization is also proposed. Accordingly, a stabledual-wavelength DFB fiber laser with a wavelength spacing

Fig. 6. Spectra of output signals at PD2 observed by the ESA under differentmodulating frequencies: �f = 10; 7; 5; 2; and 1:5 GHz.

of 0.312 nm is achieved. The beating signal of the two wave-lengths is 38.67 GHz with a 3-dB bandwidth of 6 kHz anda frequency drift 5 MHz without any feedback mechanism.As a possible application of the proposed dual-wavelengthfiber laser, a tunable microwave source ranging from 18.67 to58.67 GHz (with some discontinuity around 38.67 GHz) is fab-ricated and partially demonstrated (18.67 to 35.67 GHz). The3-dB bandwidth of the microwave signal is less than 10 kHzin the whole tuning range. Further researches towards, amongother things, increased center frequency (sub-terahertz) anddecreased frequency drift of the beating signal are expected.

ACKNOWLEDGMENT

The authors are indebted to Prof. C. Fan for his pertinent ad-vice and fruitful help in manuscript revision.

REFERENCES

[1] J. Chow, G. Town, B. Eggleton, M. Ibsen, K. Sugden, and I. Bennion,“Multiwavelength generation in an erbium-doped fiber laser usingin-fiber comb filters,” IEEE Photon. Technol. Lett., vol. 8, no. 1, pp.60–62, Jan. 1996.

[2] Y. Yao, X. Chen, Y. Dai, and S. Xie, “Dual-wavelength erbium-dopedfiber laser with a simple linear cavity and its application in microwavegeneration,” IEEE Photon. Technol. Lett., vol. 18, no. 1, pp. 187–189,Jan. 1, 2006.

[3] N. Park and P. F. Wysocki, “24-line multiwavelength operation of er-bium-doped fiber-ring laser,” IEEE Photon. Technol. Lett., vol. 8, no.11, pp. 1458–1461, Nov. 1996.

[4] M. Ibsen, E. Ronnekleiv, G. J. Cowle, M. N. Zervas, and R. I. Laming,“Multiple wavelength all-fiber DFB lasers,” Electron. Lett., vol. 36, no.2, pp. 143–144, Jan. 2000.

[5] R. Slavik, I. Castonguay, S. LaRochelle, and S. Doucet, “Short mul-tiwavelength fiber laser made of large-band distributed Fabry-Perotstructure,” IEEE Photon. Technol. Lett., vol. 16, no. 4, pp. 1017–1019,Apr. 2004.

[6] J. J. O’Reilly, P. M. Lane, R. Heidemann, and R. Hofstetter, “Opticalgeneration of very narrow linewidth millimetre wave signals,” Electron.Lett., vol. 28, no. 25, pp. 2309–2310, Dec. 1992.

[7] M. Ibsen, M. K. Durkin, M. J. Cole, and R. I. Laming, “Optimizedsquare passband fiber Bragg grating with in-band flat group delay re-sponse,” Electron. Lett., vol. 34, no. 8, pp. 800–801, Aug. 1998.